Abstract

Global distribution of platyhelminth parasites and their host specificities are not wellknown. Our hypothesis was that platyhelminth parasites of large pelagic fishes arecommon around the world. We analysed molecular variation in three different taxaof platyhelminth parasites infecting four species of tunas: yellowfin tuna (Thunnusalbacares, Scombridae) from Western Australia, southern bluefin tuna (Thunnusmaccoyii, Scombridae) from South Australia, Pacific bluefin tuna (Thunnus orientalis,Scombridae) from Pacific Mexico and northern bluefin tuna (T. thynnus, Scombridae)from two localities in the Mediterranean (Spain and Croatia). Comparisons of ITS2and partial 28S rDNA demonstrated two congeneric species of blood flukes (Digenea:Sanguinicolidae) from multiple hosts and localities: Cardicola forsteri from southernbluefin and northern bluefin tunas, and Cardicola sp. from Pacific bluefin andnorthern bluefin tunas; and a gill fluke, Hexostoma thynni (Polyopisthocotylea:Hexostomatidae), from yellowfin, southern bluefin and northern bluefin tunas.Partial 28S rDNA indicates that a second type of fluke on the gills, Capsala sp.(Monopisthocotylea: Capsalidae), occurs on both southern bluefin and Pacific bluefintunas. This appears to be the first report of conspecific platyhelminth parasites ofteleosts with a wide-ranging geographical distribution that has been confirmedthrough molecular approaches. Given the brevity of the free-living larval stage ofboth taxa of flukes on the gills (H. thynni and Capsala sp.), we conclude that the onlyfeasible hypothesis for the cosmopolitan distribution of these flatworms is migrationsof host tunas. Host migration also seems likely to be responsible for the widespreadoccurrence of the two species of blood flukes (Cardicola spp.), although it is alsopossible that these were translocated recently by the spread of infected intermediatehosts